Flat panel display assembly is a complicated process that involves coupling electronic driver chips to circuitry mounted on a glass substrate. Often flat panel displays use chip-on-glass (COG) bonding to connect row and column driver circuitry to glass mounted display electronics. COG bonding often utilizes an anisotropic conducting film (ACF) tape containing a dispersion of conducting particles held together by an adhesive. An article entitled “Micropitch connection using anisotropic conductive materials for driver IC attachment to a liquid crystal display” IBM Journal of Research and Development, Vol. 42, Numbers ¾, 1998 describes the use of ACF tape and is hereby incorporated by reference in its entirety. In particular, the article describes applying bumps of gold to the contact pads of the driver chips while the driver chips are still in wafer form. The chips are then diced from the wafer.
Applying bumps to the wafer is typically an expensive process that involves numerous process steps including sputtering thin metal films, photolithographic masking, electroplating and chemical etching. These methods are used to produce bumps at the fine pitches needed for displays with small pixels such as those used in portable devices. For coarser pitch bumps electroless plating avoids photolithographic masking and offers a simpler and less expensive method of bump deposition. However, electroless plating offers limited pitch and involves plating a substantial quantity of metal.
During assembly, the ACF tape is applied to bonding pads or contacts on the display edge. Driver chips are pressed and bonded to the other side of the ACF tape. Heat and pressure applied during bonding causes melting and flowing of the tape adhesive. Ideally, particles are sandwiched between the display contacts and the driver chip contacts to form an electrical contact. Unfortunately, the adhesive flowing can “wash out” some particles in the ACF film. The washed out particles can accumulate between adjacent pads and cause electrical shorts.
A second problem results from the limited compliance of anisotropic particles in ACF tape. Incompressible, larger anisotropic particles can create large gaps between the driver chips and a contact pad. A large gap between a display contact pad and a driver chip contact can produce open adjacent contacts between an adjacent display contact pad and a corresponding adjacent driver chip contact.
Another type of electronic interconnection is solder bumping. In solder bump wafer level packaging, a solder bump or ball is placed onto the electrical contact pads of a wafer by electroplating, screen printing or vacuum ball placement. Although solder bumps avoid ACF tape problems, solder bump fabrication involves non-recurring engineering expenses for the creation of masks, screens or vacuum receptacles.
Thus, an improved method of interconnecting chips, especially chips with bumped contacts is needed.